fix(client): wire FundCampaignModal to real useFundCampaign

[shinzoxD]

Summary

FundCampaignModal submitted through mocked campaignService.fundCampaign (setTimeout + Math.random hash) instead of the real escrow mutation. A user could see "Contribution Successful!" with a fake transaction hash without moving any tokens.

This PR:

• Wires the modal to useFundCampaign() in hooks/contract/useEscrowMutations.ts
• Uses useWallet().publicKey as the investor (drops the hardcoded GDF4...M9XZ default)
• Prompts to connect when no wallet is connected and disables submit until then
• Keeps client-side validateContribution (zero / negative / exceeds remaining target)
• Parses amounts as whole-number bigint contract units (i128), matching create-campaign / admin forms
• Removes the mock fundCampaign helper so it cannot be re-wired by a page
• Leaves campaignService.ts in place for validation and ownership-share helpers still used by the modal

Success no longer invents a fake 0x transaction hash. Mutation toasts still come from useFundCampaign; inline errors use toUserFacingError.

Distinct from:

• Unify campaign/investor pages on soil/leaf tokens and wire CampaignDetailPage to :id #197 (restyles FundCampaignModal onto soil/leaf tokens; does not wire useFundCampaign)
• fix: connect CampaignDetailPage to real contract data and route CreateCampaignPage (Resolves #186) #206 (CampaignDetailPage :id)
• feat(client): add dispute open/resolve UI #183 (OpenDisputeForm / dispute UI)

Closes #139

Test plan

• Submit path mocks useFundCampaign and asserts { campaignId, investor, amount: 500n }
• Invalid / over-target amounts never call the mutation
• Disconnected wallet shows a connect prompt and disables submit
• Success path does not render a fake 0x tx hash
• Contract rejection surfaces a user-facing error
• Existing FundCampaignModal accessibility tests still pass
• campaignService validation tests converted to vitest (no fake hash assertions)

cd client
npx vitest run src/components/campaign/__tests__/FundCampaignModal.test.tsx src/__tests__/campaignService.test.ts src/__tests__/accessibility.test.tsx

20 passed.

GrantFox claim already posted. Wallet: GBVHELLD2JE235Y2NGTDT3MWI3T65ON6SY4N6FBHYVDAQ5FZC2CP5QXH

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[shinzoxD]

Closing this PR. GrantFox assigned #139 to @Mosas2000 after I opened it — I won't compete with the assigned contributor.